How to unit test a callback in parameter - Kotlin

Question

I have a Client class (written in Kotlin in an Android app) that implements an interface ReadyCallback (written in Java in a library of the app, the app is dependent on this library). In Client I have a createClient() method which will create a client with the parameter of ReadyCallback. If it's ready, I will perform other tasks by calling classC.otherMethod(), if not ready, I just create the client without doing other stuff:

In the library:

// Somewhere in this library, I have logic to call `readyCallback.onReady()` when I consider it's "ready"
interface ReadyCallback {
    void onReady()
}

class Manager {
    private final ReadyCallback readyCallback;
    public void onConnected(final boolean isConnected) {
        if (isConnected) {
            readyCallback.onReady();
        }
    }
}

In the app:

class ClassA internal constructor(private val clientProvider: ClassB, private val classC: ClassC, private val classD: ClassD) : ReadyCallback {
    fun createClient() {
        val client = clientProvider.create(getReadyCallback())
    }

    private fun getReadyCallback() {
        return ReadyCallback { onReady() }
    }

    override fun onReady() {
        logInfo { "It's ready! Now do some stuff by calling classC.otherMethod()" }
        classC.otherMethod()
    }
}

In unit test, I want to verify that when I create the client and it's ready, classC's otherMethod() will be invoked. I tried to do the following but it's not correct:

import com.nhaarman.mockitokotlin2.*
import org.junit.*

class ClassATest {
    lateinit var unitUnderTest: ClassA
    lateinit var clientProviderMock: ClassB
    lateinit var classCMock: ClassC
    lateinit var clientMock: ClassD

    @Before
    override fun setup() {
        super.setup()
        clientProviderMock = mock()
        classCMock = mock()
        clientMock = mock()
        unitUnderTest = ClassA(clientProvider = clientProviderMock, classC = classCMock, classD = classDMock)

        whenever(clientProviderMock.create(any()).thenReturn(client)
    }

    @Test
    fun `when create client then call otherMethod`() {
        unitUnderTest.createClient()
        verify(classCMock).otherMethod()
    }
}

The error message shows:

Wanted but not invoked:
classC.otherMethod();
Actually, there were zero interactions with this mock.

I think the reason I got this error is because, if I don't call getReadyCallback(), it means I am not invoking the callback, so there's no call to classC.otherMethod(). But other than that I am really stuck on this, I don't know how to unit test my desire behavior (If it's ready, classC.otherMethod() will be called, if not ready, this method won't be called).

I know I can't do things like below because unitUnderTest is not a mock object:

callbackMock = mock()
whenever(unitUnderTest.getReadyCallback()).thenReturn(callbackMock)
whenever(clientProviderMock.create(callbackMock).thenReturn(client)

Can anyone help me out please?

Hi there. Can you update your question to include the log output, and the relevant dependencies (Kotlin, Mockito, Junit, etc) you've got in your Gradle/Maven file, including the versions. It's important to check the versions are lining up. Also the code you've provided has errors (missing brackets, `void onReady()` isn't Kotlin, classes B/C/D are missing) so it's hard to check - can you update it to make sure it's [reproducible](https://stackoverflow.com/help/minimal-reproducible-example)? — aSemy, Sep 13 '21 at 06:17
In the test setup you stub `clientProvider.create(...)`, so in `ClassA.createClient()` it's never going to call `getReadyCallback()` - instead Mockito intercepts `clientProvider.create(...)` and will immediately return the mock client. — aSemy, Sep 13 '21 at 06:32
Sorry I didn't make it clear. Yes it is Java not Kotlin, it's from another package that classA's package depend on. I tried to edit the question. — peanutnut, Sep 13 '21 at 13:57

peanutnut · Answer 1 · 2021-09-14T01:16:19.637

The only way I can think of is to add a boolean flag in callback's onReady() method. So it will become:

In library:

interface ReadyCallback {
    void onReady(final boolean isReady)
}

class Manager {
    private final ReadyCallback readyCallback;
    public void onConnected(final boolean isConnected) {
        if (isConnected) {
            readyCallback.onReady(true);
        } else {
            readyCallback.onReady(false);
        }
    }
}

In app:

class ClassA internal constructor(private val clientProvider: ClassB, private val classC: ClassC, private val classD: ClassD) : ReadyCallback {
    fun createClient() {
        val client = clientProvider.create(getReadyCallback())
    }

    private fun getReadyCallback() {
        return ReadyCallback { isReady -> onReady(isReady) }
    }

    override fun onReady(isReady: Boolean) {
        if (isReady) {
            logInfo { "It's ready! Now do some stuff by calling classC.otherMethod()" }
            classC.otherMethod()
        }
    }
}

In unit test:

import com.nhaarman.mockitokotlin2.*
import org.junit.*

class ClassATest {
    lateinit var unitUnderTest: ClassA
    lateinit var clientProviderMock: ClassB
    lateinit var classCMock: ClassC
    lateinit var clientMock: ClassD

    @Before
    override fun setup() {
        super.setup()
        clientProviderMock = mock()
        classCMock = mock()
        clientMock = mock()
        unitUnderTest = ClassA(clientProvider = clientProviderMock, classC = classCMock, classD = classDMock)

        whenever(clientProviderMock.create(any()).thenReturn(client)
    }

    @Test
    fun `when create client and ready then call otherMethod`() {
        unitUnderTest.onReady(true)
        unitUnderTest.createClient()
        verify(classCMock).otherMethod()
    }

    @Test
    fun `when create client and not ready then do not call otherMethod`() {
        unitUnderTest.onReady(false)
        unitUnderTest.createClient()
        verifyZeroInteractions(classCMock)
    }
}

But I still don't know how to test without the boolean parameter in the callback's method. Does anyone know how to do that?

`otherMethod()` will never be called (unless you directly call `onReady()`) because it's wrapped inside a lambda, `ReadyCallback`, and the lambda is ignored and never executed because `clientProvider` is a mock. Try writing the test without mocks and debug step through the code - hopefully that makes it clear. — aSemy, Sep 13 '21 at 21:08

score 0 · Accepted Answer · answered Sep 15 '21 at 02:15

I think I figured it out. I don't need a parameter in onReady(). In library:

interface ReadyCallback {
    void onReady()
}

// place to determine when is "ready"
class Manager {
    private final ReadyCallback readyCallback;
    public void onConnected(final boolean isConnected) {
        if (isConnected) {
            readyCallback.onReady();
        }
    }
}

In app:

class ClassA internal constructor(private val clientProvider: ClassB, private val classC: ClassC, private val classD: ClassD) : ReadyCallback {
    fun createClient() {
        val client = clientProvider.create(getReadyCallback())
    }

    private fun getReadyCallback() {
        return ReadyCallback { onReady() }
    }

    override fun onReady() {
        logInfo { "It's ready! Now do some stuff by calling classC.otherMethod()" }
        classC.otherMethod()
    }
}

In unit test:

import com.nhaarman.mockitokotlin2.*
import org.junit.*

class ClassATest {
    lateinit var unitUnderTest: ClassA
    lateinit var clientProviderMock: ClassB
    lateinit var classCMock: ClassC
    lateinit var clientMock: ClassD

    @Before
    override fun setup() {
        super.setup()
        clientProviderMock = mock()
        classCMock = mock()
        clientMock = mock()
        unitUnderTest = ClassA(clientProvider = clientProviderMock, classC = classCMock, classD = classDMock)

        whenever(clientProviderMock.create(any()).thenReturn(client)
    }

    @Test
    fun `when create client and ready then call otherMethod`() {
        unitUnderTest.onReady()
        unitUnderTest.createClient()
        verify(classCMock).otherMethod()
    }

    @Test
    fun `when create client and not ready then do not call otherMethod`() {
        unitUnderTest.createClient()
        verifyZeroInteractions(classCMock)
    }
}

Feel free to mark this as the best answer if you have it solved. — Alex, Sep 15 '21 at 04:22
It doesn't look like this actually tests your code. It's hard to tell because the code you've provided doesn't compile, but if you change `return ReadyCallback { onReady() }` to `return ReadyCallback { }` then it your tests will still pass. It also looks like `unitUnderTest.createClient()` in test `when create client and ready then call otherMethod` doesn't do anything and can be removed. You need to replace the mocks with actual instances - because you're stubbing methods and so your code isn't executing during a test. — aSemy, Sep 18 '21 at 15:53

How to unit test a callback in parameter - Kotlin

2 Answers2